Materials Notebook: Hot and Cold Recycling of Asphalt Pavements
Hot and Cold Recycling of Asphalt Pavements
FHWA N. 5080.93
October 6, 1981
To present the Federal Highway Administration (FHWA) position on recycling of asphalt pavements.
The FHWA Technical Advisory T5040.9 dated February 16, 1979, Hot Recycling of Asphalt Pavement Materials, is canceled.
The pressing need to conserve energy and minimized costs in highway construction requires that special effort be made to identify and make the maximum use of procedures that will result in reduced energy usage and minimum costs. Because recycling of asphalt pavements has the potential to be an effective method of conserving energy and materials and reducing costs, it is FHWA's policy that recycled asphalt concrete, defined as asphalt concrete containing salvaged paving materials including the use of suitable reclaimed material from other projects, be allowed tor use on all projects. States with insufficient experience to properly evaluate the reuse of these materials should take immediate steps to initiate experimental projects.
- Recycled hot asphalt concrete is an asphalt concrete mix, processed hot in a central plant, which consists of sized salvaged asphalt material, new asphalt, and/or recycling agents and new and/or salvaged aggregates, and meets all standard material and mix specifications for the type of mix being produced.
- Recycled cold mix is an asphalt concrete mix, processed in a central plant or on the grade which consists of sized salvaged asphalt material, some type of stabilizing agent and new and/or salvaged aggregates. This material meets specifications of an asphalt aggregate base and generally requires that an asphalt surface course or surface seal be used.
- PAVEMENT DESIGN
- Recycling should be one of the options considered at the design stage of all rehabilitation projects. Material testing of the old pavement may be necessary to determine that recycling is a practical option. The decision to recycle or to overlay should be based on cost and performance on a life cycle basis rather than initial cost and should be specified by the contracting agency. It is emphasized that alternate bids between recycling and overlay are not recommended.
- Cracks and material deficiencies in the overlaid pavement will cause reflective cracks and points of weakness to occur in an overlay. Cracks can be eliminated and material deficiencies can be corrected by recycling.
- Recycled mixes placed experimentally as base layers, top structural layers, and wearing surfaces are still being evaluated and it would be premature to offer definite conclusions on life cycle performance. However, the earliest of those pavements are 5 years old or older and are performing as well as pavements constructed with new materials. While there is limited experience with recycled mixes, it appears that reasonable performance can be obtained.
- It is reasonable to assume that a recycled layer is structurally equivalent to an equal thickness of new hot mix pavement provided the mix meets all of the laboratory design criteria for a new mix intended to perform the same functions.
- Only proven methods and materials with which there has been adequate experience to assure success should be used on large projects with high traffic or heavy loading.
- MIX DESIGN
- Recommendations for detailed mix design procedures are contained in NCHRP Report 224. Gradation and other material requirements should be the same for a recycled mix as those developed for mixes using all new materials for the same type of pavement.
- Distress observed on a few projects is directly attributable to improper or poor mix designs represented by low stabilities, uncorrected aggregate stripping problems, and low job achieved densities. These problems emphasize the need for proper mix design and construction control. Research results indicate that testing for water susceptibility is especially important for recycled mixes.
- Variation in material properties of the pavements to be salvaged should be identified by sampling and a sufficient proportion of new material provided to reduce the variation to an acceptable level. Major changes in mix characteristics for various sections along the same route usually demand separate mix designs.
- Removal and sizing of salvaged pavement materials have at times created additional minus 200 sieve material. The amount depends on the type and operation of the sizing process and aggregate properties. Final mix design should always be corrected to final properties of the material processed by the actual equipment used on the project. Large amounts of minus 200 sieve material or other gradation deficiencies can be compensated for by limiting the amount of salvaged material used in the recycled mix and varying the gradation of the added new material. Experience has indicated that in most cases crushing the recycled material to a maximum particle size of 2 inches is adequate for hot mix. Additional crushing may result in excess fines.
- A soft asphalt alone has been used successfully to restore the penetration and viscosity of the reclaimed asphalt binder. A number of commercial recycling agents have also been used when salvaged asphalt binder in the salvaged material was severely hardened. Any proposed softening agent should be tested with the salvaged asphalt for the specific project, in the ratio to be used, to assure the desired properties of the combination are realized.
- REMOVAL AND SIZING
The type and degree of deterioration in a pavement to be constructed and/or the type of material underlying the pavement will usually determine whether a full or partial depth removal technique is utilized. Full-depth pavement removal and sizing can be accomplished using standard construction equipment such as dozer and loaders and portable or stationary crushers or by milling machines. The latter process, although generally more expensive, allows removal of one lane without disturbing traffic movement on adjacent lanes. Excessive dropoffs can be minimized by milling successive levels to a specific depth. While milling machines usually are specified for partial depth removal, the choice of the method used for full-depth removal will be influenced by economics and maintenance of traffic through construction.
Virtually all equipment manufactured today for the production of asphalt concrete can be built to produce acceptable recycled mixes and meet all air quality standards. Existing equipment can be modified at reasonable cost. In hot mix recycling, batch plants are generally limited to the reuse of a maximum amount of 50 percent salvaged asphalt material in a recycled mix, while an upper limit of approximately 70 percent is attainable in some drum plants.
Materials savings are realized from the reduction in new asphalt and aggregate. Energy savings result primarily from reduced aggregate haul and drying, and asphalt transportation. Coat savings are greatly influenced by length of aggregate haul and distance from the plant to the job site. Other factors which have a mayor influence on bid prices are the degree to which contractors in the area are familiar with and equipped for recycling, the size of the State's present and projected recycling program, and State contract procedures.
- Allow the contractor the use of salvaged asphalt materials and aggregates in the production of asphalt concrete.
- Allow the contractor to determine the source and amount of salvaged material to be used as long as the mix produced meets all standard material and mix specifications called for in the contract.
- Require that a revised mix design be submitted and approved prior to changing either the source or amount of salvaged material originally approved.
- Serious consideration should be given to transferring ownership of all material to be removed to the contractor. This allows the owner agency to receive instant credit, in the form of lower bids, for the value of the salvaged material removed.
- Do not specify how to remove and size a pavement scheduled for full-depth reconstruction; what type of hot mix plant (batch, continuous or drum) to use; the use of recycling agent--but allow it to be used; and what percentage of salvaged material to be used. All of these will be determined by economics resulting from the competitive bidding process.
- Recycled hot asphalt concrete should be paid for on the basis of a bid price per ton regardless of the percentage of salvaged material used. This price per ton is also to include the costs of all new additional asphalt, recycling agent, and aggregate.
- DISCUSSION OF RECOMMENDATIONS
These recommended practices will allow the production of recycled asphalt concrete, if economically feasible at any time in any location. Because no restrictions are placed on percentages of used salvaged material, a batch plant owner can economically compete with owners of drum plants. If across the board use of salvaged materials is allowed in the production of asphalt concrete, the contracting industry can better justify gearing up for such production and write off the additional plant modification cost over a much larger tonnage basis over a longer period of time than on only one or two projects.
Transferring ownership of all removed salvaged material to the contractor encourages recycling because surplus material can be used in private work at additional savings to the contractor.
Most highway agencies have successfully constructed one or more hot recycling projects and are continuing to develop new projects. These projects have been constructed under NEEP Project 22, Pavement Recycling, distributed by Notice N 5080.64 dated June 3, 1977. Many projects have also been constructed with technical and financial assistance from the Demonstration Project program. It is recommended that the evaluation of these projects be continued to validate long-term performance. Broad participation is needed to provide the data base necessary to require additional projects to be programmed experimental. The projects under evaluation should be representative of recycling procedures adopted by a State which have become routine. When a significantly new or innovative feature is contemplated, or when a project is in a significantly different environment, the highway agency should be urged to designate the project as experimental.
A recycling data bank is being developed under a contract through the FHWA Office of Research that will provide a means of long term evaluation of pavement recycling. The contract is scheduled to be completed in 1982.
SIGNED R.D. MORGAN
Associate Administrator for Engineering and Traffic Operations